This invention relates to devices for measuring or diplaying an electrical characteristic of living tissue such as that of a human being or animal. Various methods and machines have been used in the past to measure and/or display certain electrical characteristic of living tissue. The purposes of such measurements included detection of increases or decreases in, for example, resistance of the skin, which were shown to occur during fluctuations in autonomic nervous system activity. Transient decreases were used by physiologists and pyschologists as indicators of increased autonomic function and anxiety. Clinical neurophysiologists on the other hand have measured for increases which would help them localize areas of skin in which the normal nerve supply had been damaged or disrupted.
For example, in the paper published in 1945 by Curt P. Richter entitled "Instructions For Using The Cutaneous Resistance Recorder or "Dermohmeter" On Peripheral Nerve Injuries, Sympathectomies, and Paravertebral Blocks" it was recognized that the resistance offered to the passage of a minute direct current through the body is localized almost entirely in the skin. A puncture made through the skin with a hypodermic needle reduces the resistance from any level practically to zero, no matter how high it was for the previously intact skin. The resistance over an abrasion or cut is likewise low. It was also recognized that skin resistance is controlled largely through the nervous system. The paper describes a small portable dermohmeter consisting of a micrommeter a 41/2 volt battery, potential divider and a telephone plug socket furnished with a jack switch. Two silver-plated phosphor-bronze electrodes were connected by an insulated line to the panel of the dermometer. One of these electrodes was constructed so that it could be clipped to an ear lobe while the other had the shape of a hammer with a large disc at one end of the head and a small disc at the opposite end. The current from the battery passed from one electrode to the other by travelling through the body of the patient and through the microammeter. The potential divider was used to regulate the amount of current that flowed from the battery through the patient. With the potential fixed, the amount of current registered by the microammeter depended on the resistance offered by the patient's skin to the passage of the current. It was suggested in the paper that the patient should be prepared before use of the instrument by exposing him to external heat for a few minutes. It was also necessary to prepare the patient by cleansing the ear lobe and pricking the skin thereat with a hypodermic needle to eliminate the skin resistance. A special paste or electrode jelly also had to be used on the electrode that was to be fastened to the ear lobe.
Also in a paper published in 1945 by Captain H. Jasper and entitled "An Improved Clinical Dermohmmeter" a different type of dermohmeter, which was recommended for Canadian service, is described. It was mounted in a wooden box measuring approximately eight inches in each direction and weighing seven pounds. It requires the use of nine standard 11/2 volt flashlight batteries, a 0-50 microammeter with a special contact, a special switch and seven fixed resistors. The specifications in the paper indicate that a current of up to 500 microamperes could be reached as the voltage is increased.
U.S. Pat. No. 2,799,269 illustrates an electropsychometer having two meters one of which registers continuously the electrical ohmic resistance existing between the skin contacting elements of a special electrode. This electrode had separate cylindrical elements mounted on a non-conductive member and the entire electrode structure was capable of being held in one hand. According to the patent, the described instrument is capable of assessing certain physical conditions such as the existence of severe emotional tension.
U.S. Pat. No. 3,834,374 to M. Ensanian describes an instrument said to be capable of diagnostic electrical scanning. A stationary electrode is placed in contact with the skin surface of the patient and then the body surface is scanned along a continuous path or over an area by rolling an exploratory electrode over the surface. This permits the instrument to map the potential along the line or over the area contacted by the exploratory electrode.
U.S. Pat. No. 3,870,034 which issued to Cyborg Corporation describes a device intended to be worn on one's wrist and capable of measuring the galvanic skin response of the wearer and thereby indicating the individual's tension level. The instrument houses an oscillator circuit, a battery, a sound emitting device and a pair of spaced electodes located on opposite ends of the instrument casing. In use the electrodes are touched by two fingers of the wearer to provide a reading of the tension level.
There are certain problems and difficulties encountered with use of some of the known devices for measuring an electrical characteristic of living tissue such as skin. Some of these known methods are considered unsafe, particularly in certain circumstances or with certain patients and they can be painful or otherwise invasive. The levels of test current required can be too high, particularly for some potential or known possible applications for these devices. Many of these known devices and methods require that the current pass through the body and this can cause undesirable effects on the body. Such effects may be so serious as to render the method too dangerous for general use, particularly on humans. As indicated earlier, some of the known methods require that the skin be punctured under one of the two electrodes with a needle or lancet set and this of course can cause discomfort. If electrically conductive creams or pastes must be used on the skin, there is a potential problem of a hypersensitive or allergic reaction in the patient.
It is an object of the present invention to provide a diagnostic device for assessing pain, abnormal sensation or sympathetic dysfunction in a human being or animal which avoids some or all of the aforementioned disadvantages and risks with the use of previously known instruments.
It is a further object of the present invention to provide a diagnostic device capable of operating on a low voltage power source, such as a battery and able to measure and quantify the level of conductance of living tissue such as human skin.